[PATCH RFC 11/11] rust: sync: arc: Add lockdep integration

Asahi Lina
2023-07-14T09:31:46Z

Now that we have magic lock class support and a LockdepMap that can be
hooked up into arbitrary Rust types, we can integrate lockdep support
directly into the Rust Arc<T> type. This means we can catch potential
Drop codepaths that could result in a locking error, even if those
codepaths never actually execute due to the reference count being
nonzero at that point.

Signed-off-by: Asahi Lina <lina@asahilina.net>
---
lib/Kconfig.debug | 8 ++++++
rust/kernel/init.rs | 6 +++++
rust/kernel/sync/arc.rs | 71 ++++++++++++++++++++++++++++++++++++++++++++++---
3 files changed, 82 insertions(+), 3 deletions(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index fbc89baf7de6..ff4f06df88ee 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -3010,6 +3010,14 @@ config RUST_BUILD_ASSERT_ALLOW

If unsure, say N.

+config RUST_EXTRA_LOCKDEP
+ bool "Extra lockdep checking"
+ depends on RUST && PROVE_LOCKING
+ help
+ Enabled additional lockdep integration with certain Rust types.
+
+ If unsure, say N.
+
endmenu # "Rust"

endmenu # Kernel hacking
diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index f190bbd0bab1..b64a507f0a34 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -1208,6 +1208,7 @@ pub trait InPlaceInit<T>: Sized {
/// type.
///
/// If `T: !Unpin` it will not be able to move afterwards.
+ #[track_caller]
fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
where
E: From<AllocError>;
@@ -1216,6 +1217,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
/// type.
///
/// If `T: !Unpin` it will not be able to move afterwards.
+ #[track_caller]
fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Pin<Self>>
where
Error: From<E>,
@@ -1228,11 +1230,13 @@ fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Pin<Self>>
}

/// Use the given initializer to in-place initialize a `T`.
+ #[track_caller]
fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
where
E: From<AllocError>;

/// Use the given initializer to in-place initialize a `T`.
+ #[track_caller]
fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
where
Error: From<E>,
@@ -1277,6 +1281,7 @@ fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>

impl<T> InPlaceInit<T> for UniqueArc<T> {
#[inline]
+ #[track_caller]
fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
where
E: From<AllocError>,
@@ -1291,6 +1296,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
}

#[inline]
+ #[track_caller]
fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
where
E: From<AllocError>,
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index a89843cacaad..3bb73b614fd1 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -34,6 +34,9 @@
};
use macros::pin_data;

+#[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
+use crate::sync::lockdep::LockdepMap;
+
mod std_vendor;

/// A reference-counted pointer to an instance of `T`.
@@ -127,6 +130,17 @@ pub struct Arc<T: ?Sized> {
_p: PhantomData<ArcInner<T>>,
}

+#[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
+#[pin_data]
+#[repr(C)]
+struct ArcInner<T: ?Sized> {
+ refcount: Opaque<bindings::refcount_t>,
+ lockdep_map: LockdepMap,
+ data: T,
+}
+
+// FIXME: pin_data does not work well with cfg attributes within the struct definition.
+#[cfg(not(CONFIG_RUST_EXTRA_LOCKDEP))]
#[pin_data]
#[repr(C)]
struct ArcInner<T: ?Sized> {
@@ -159,11 +173,14 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}

impl<T> Arc<T> {
/// Constructs a new reference counted instance of `T`.
+ #[track_caller]
pub fn try_new(contents: T) -> Result<Self, AllocError> {
// INVARIANT: The refcount is initialised to a non-zero value.
let value = ArcInner {
// SAFETY: There are no safety requirements for this FFI call.
refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
+ #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
+ lockdep_map: LockdepMap::new(),
data: contents,
};

@@ -178,6 +195,7 @@ pub fn try_new(contents: T) -> Result<Self, AllocError> {
///
/// If `T: !Unpin` it will not be able to move afterwards.
#[inline]
+ #[track_caller]
pub fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Self>
where
Error: From<E>,
@@ -189,6 +207,7 @@ pub fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Self>
///
/// This is equivalent to [`Arc<T>::pin_init`], since an [`Arc`] is always pinned.
#[inline]
+ #[track_caller]
pub fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
where
Error: From<E>,
@@ -292,15 +311,46 @@ fn drop(&mut self) {
// freed/invalid memory as long as it is never dereferenced.
let refcount = unsafe { self.ptr.as_ref() }.refcount.get();

+ // SAFETY: By the type invariant, there is necessarily a reference to the object.
+ // We cannot hold the map lock across the reference decrement, as we might race
+ // another thread. Therefore, we lock and immediately drop the guard here. This
+ // only serves to inform lockdep of the dependency up the call stack.
+ #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
+ unsafe { self.ptr.as_ref() }.lockdep_map.lock();
+
// INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
// this instance is being dropped, so the broken invariant is not observable.
// SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
+
if is_zero {
// The count reached zero, we must free the memory.
- //
- // SAFETY: The pointer was initialised from the result of `Box::leak`.
- unsafe { Box::from_raw(self.ptr.as_ptr()) };
+
+ // SAFETY: If we get this far, we had the last reference to the object.
+ // That means we are responsible for freeing it, so we can safely lock
+ // the fake lock again. This wraps dropping the inner object, which
+ // informs lockdep of the dependencies down the call stack.
+ #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
+ let guard = unsafe { self.ptr.as_ref() }.lockdep_map.lock();
+
+ // SAFETY: The pointer was initialised from the result of `Box::leak`,
+ // and the value is valid.
+ unsafe { core::ptr::drop_in_place(&mut self.ptr.as_mut().data) };
+
+ // We need to drop the lock guard before freeing the LockdepMap itself
+ #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
+ core::mem::drop(guard);
+
+ // SAFETY: The pointer was initialised from the result of `Box::leak`,
+ // and the lockdep map is valid.
+ #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
+ unsafe {
+ core::ptr::drop_in_place(&mut self.ptr.as_mut().lockdep_map)
+ };
+
+ // SAFETY: The pointer was initialised from the result of `Box::leak`, and
+ // a ManuallyDrop<T> is compatible. We already dropped the contents above.
+ unsafe { Box::from_raw(self.ptr.as_ptr() as *mut ManuallyDrop<ArcInner<T>>) };
}
}
}
@@ -512,6 +562,7 @@ pub struct UniqueArc<T: ?Sized> {

impl<T> UniqueArc<T> {
/// Tries to allocate a new [`UniqueArc`] instance.
+ #[track_caller]
pub fn try_new(value: T) -> Result<Self, AllocError> {
Ok(Self {
// INVARIANT: The newly-created object has a ref-count of 1.
@@ -520,13 +571,27 @@ pub fn try_new(value: T) -> Result<Self, AllocError> {
}

/// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
+ #[track_caller]
pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
// INVARIANT: The refcount is initialised to a non-zero value.
+ #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
+ let inner = {
+ let map = LockdepMap::new();
+ Box::try_init::<AllocError>(try_init!(ArcInner {
+ // SAFETY: There are no safety requirements for this FFI call.
+ refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
+ lockdep_map: map,
+ data <- init::uninit::<T, AllocError>(),
+ }? AllocError))?
+ };
+ // FIXME: try_init!() does not work with cfg attributes.
+ #[cfg(not(CONFIG_RUST_EXTRA_LOCKDEP))]
let inner = Box::try_init::<AllocError>(try_init!(ArcInner {
// SAFETY: There are no safety requirements for this FFI call.
refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
data <- init::uninit::<T, AllocError>(),
}? AllocError))?;
+
Ok(UniqueArc {
// INVARIANT: The newly-created object has a ref-count of 1.
// SAFETY: The pointer from the `Box` is valid.

--
2.40.1